The present embodiments relate to a detuning apparatus for a receive coil for a magnetic resonance device.
Magnetic resonance imaging is known in the prior art. Magnetic resonance imaging uses coil elements of a transmit coil to excite nuclear spins in a target object (e.g., a certain layer of the target object). The resulting magnetic resonance signals may be received by coil elements of at least one receive coil that are resonant at this frequency to allow receipt. During transmission by way of the transmit coil, coil elements of the receive coil are held in a detuned state, so that the transmit signal is not injected at the coil elements, having a negative impact in the process. A detuning assembly (e.g., a PIN diode) is provided in the coil element, by way of which the coil element may be dynamically detuned at least during the transmit process.
Increasingly frequent use is made of receive coils that have a large number of coil elements, not all of which are to be read out or may be read out for a measurement. A switching apparatus connects receive coil elements to be read out to corresponding receive channels of a data processing device (e.g., an evaluation device). Coil elements may be connected to receive channels in any manner. If, for example, due to the technically limited size of the homogeneity volume of the magnetic resonance device, a magnetic resonance examination is performed in a number of acts, in which different regions of the target object are recorded, only the coil elements that are intended to receive in the region being examined in each instance, dynamically between a resonant receive state and a non-resonant detuning state (e.g., during the transmit phase with the transmit coil), may be switched during a sequence. Other coil elements that are not contributing to the imaging process at the time may be statically detuned.
Different options are known in the prior art for specifically achieving such a detuning capacity. Three of the options are set out in more detail below.
In one embodiment, each coil element of the receive coil may have a cross-connected diode pair that blocks in the receive instance (e.g., at very low power) and becomes low-resistance in the high-frequency transmit instance (e.g., at very high high-frequency power). Such idling or short circuit is used by way of a quarter lambda line to tune or detune the receive element. This solution is disadvantageous in that at low transmit powers in some circumstances the diodes do not become completely low-resistance. The precise high-frequency transmit level, at which the transition takes place from a non-conducting to a conducting state of the diodes, also is a function of the location, size and charge of the receive coil. It may therefore occur that the receive element is not detuned in time and therefore influences the homogeneity of the high-frequency transmit field that may result in reduced image quality.
In another embodiment, each receive coil element is provided with a PIN diode that is connected by an appropriate line to a controlled current/voltage source. The current/voltage source allows the PIN diode to be switched to a low-resistance or high-resistance state. The high-resistance state, which is present when a voltage of, for example, 40 V is applied, may be used for the receive process, since the noise is much lower than in a state in which current of, for example, 30 mA flows through the PIN diode. This state is used as the detuning state. To detune the PIN diode, therefore, a certain current flow is required. The required current flow may also be supplied by the current/voltage source.
The coil elements used for receiving during a measurement may be dynamically tuned and detuned. This is done using a shared dynamic activation signal, by way of which the current/voltage sources are dynamically activated. Static detuning is maintained for the coil elements, which are not to be used to receive magnetic resonance signals (e.g., the current/voltage sources do not respond to the dynamic control signal). The current/voltage sources may have a further control input, for example, that is additionally activated via a controller. A register may be provided as, for example, part of a controller that establishes whether a current/voltage source responds to the dynamic signal and which coil elements are connected by way of the switching apparatus to the data processing apparatus so that the data processing apparatus may evaluate the receive signal.
In this embodiment, the current/voltage sources are disposed outside the examination volume (e.g., outside the receive coil on the main magnet unit or in a technical area). This has two disadvantages. The electric power required to switch the PIN diode is to be transmitted via a connecting line between the current/voltage source and the PIN diode, so that corresponding requirements result for the connecting lines with respect to cross-sectional design, impedance, blocking and the like. A separate connecting line is used for every PIN diode to be controlled.
The choice of which coil elements are switched to which receive channels and which PIN diodes are detuned statically or dynamically is made separately in each instance and forwarded. The switchable current/voltage sources and the switches of the switching apparatus are each activated via separate control lines, making the system extremely complex.
To simplify the system, the cabling (e.g., separate cables previously used for the high-frequency receive signals and the PIN diode activation signals) was reduced by transmitting the signals at least to some extent on a shared line using frequency multiplexing. The activation signals for the PIN diodes after the switching apparatus are also forwarded onto the lines to the coil elements. The magnetic resonance receive signal may have a frequency that is >5 MHz, but the PIN activation signal is a switching signal with a switching time of approximately 10 μs.
In yet another embodiment, in contrast to the embodiment above, the current/voltage source is not provided outside the examination volume but is provided directly on the coil element. Since this is extremely close to the object to be examined (e.g., a patient), the current/voltage source is to be small and designed with as little power loss as possible. For example, a high negative supply voltage (e.g., −40 V) as the voltage source and a lower positive supply voltage (e.g., 5 V) with a correspondingly dimensioned pre-resistor as the current source may be switched between. This arrangement has the advantage that the energy used to switch the PIN diode may not be transmitted by way of the sometimes very long connecting lines. Only a logic signal may run on the line.
In addition to the arrangements described here, further solutions that have not found their way into an actual product (e.g., detuning by way of an auxiliary carrier that is emitted using the whole-body antenna, or detuning via an optically controlled semiconductor switch that is switched by the measurement controller via a glass fiber) are known. These methods are technically difficult to implement.